DE-OS No. 23 11 635 discloses a method for the continuous manufacture of dispersed polyurethane urea materials, in accordance with which a prepolymer carrying isocyanate end groups, a chain-extending agent, and a liquid aqueous phase are introduced continuously and individually into a zone of high turbulence. The isocyanate polyaddition into polyurethane urea is effected in the zone of high turbulence. A polyurethane urea dispersion in the form of fine particles is produced. The method is complicated and leads to high-melting products which are, therefore, unsuitable for many purposes. In addition, the polyurethane urea materials produced have a very high urea group content.
Polyurethane urea materials with melting points below about 200.degree. C. and, in particular, in the range of from about 100.degree. to 150.degree. C. cannot be prepared by the above-mentioned method. However, polyurethane ureas having melting points below about 200.degree. C. are well suited for many purposes. For example, they are useful as adhesives for insert materials which can be ironed-in. When the polyurethane urea material is to be employed as an adhesive, it is desirable to prepare the polyurethane urea in a finely dispersed or powdered form. The powders can then be used with one of the conventional application methods, i.e., by sprinkling-on or by printing-on the adhesive via a dispersion.
At first glance it may appear feasible to mechanically comminute or mill known polyurethane ureas having the above-indicated melting range in order to manufacture polyurethane urea adhesive powders. Such a milling process is described by DE-OS No. 19 30 340. However, the milling process disclosed by this reference has practical disadvantages. During milling the granulate becomes quite hot and requires cooling, for example, with liquid nitrogen. Moreover, the product particles produced by this process have a greatly differing morphology, with regard to size and shape.
It has further been suggested to produce polyurethane powders by reacting the starting materials in an organic solvent or mixture of solvents, which will not function as a solvent for the reacted polyurethanes or polyurethane ureas. (See DE-OS No. 26 46 198, U.S. Pat. No. 4,032,516, DE-OS No. 25 56 945) A disadvantage of this process, however, is that a large amount of solvent is employed which must be recovered at considerable cost. Moreover, the use of solvents requires arrangements for the safety of work rooms, and for the observation of regulations for preventing the pollution of the environment.